Various approaches have been considered for skin rejuvenation and other tissue/skin treatments based on delivery of energy to the tissue being treated. In non-ablative approaches, energy is delivered to tissue, but no tissue is thereby removed. In ablative approaches, energy is delivered to tissue such that some of the tissue is removed by ablation.
Non-ablative skin rejuvenation (e.g., using radiofrequency, ultrasound, or light) is typically performed using application of the electrical current, ultrasound energy or light beam to the tissue surface, and heating tissue to temperatures not exceeding the vaporization threshold. Optical examples of this approach include U.S. Pat. No. 6,723,090. Electrical examples of this approach include U.S. Pat. Nos. 5,871,524; 6,662,054.
Carbon dioxide (CO2) laser systems have been recently applied to ablative fractional resurfacing of human skin. In these procedures, a pulsed CO2 laser is used to drill channels of approximately 100 micrometers in diameter and 0.5-0.7 mm in depth. Using scanning mirror, these holes are applied in patterns with spacing of approximately 1 mm. The epidermis and part of the dermis demonstrate columns of thermal coagulation that surround tapering ablative zones lined by a thin eschar layer. Typically, a thermal coagulation zone at the edges of such laser channels in skin is on the order of 40 micrometers. Such ablation (tissue removal) and coagulation of skin leads to stimulation of its rejuvenation and tightening, and thus results in improved cosmetic appearance. This approach, called “fractional skin resurfacing” was found to be clinically very effective. An example of this approach is considered by Hantash et al. in an article titled “In vivo Histological Evaluation of a Novel Ablative Fractional Resurfacing Device” (Lasers in Surgery and Medicine 39:96-107 (2007)).
Disadvantages of the CO2 laser systems include their relatively large size, somewhat cumbersome articulated arm beam delivery system, and relatively high cost. In addition, since ablation craters produced by lasers taper towards the bottom, there is a limit on the ratio of depth-to-width of the channels (so called aspect ratio) that can be produced by laser ablation. Typically this aspect ratio does not exceed 10, i.e. channels of 100 μm in diameter do not exceed 1 mm in depth. Another limitation of the laser-based tissue drilling approach is that the thermal damage zone at the side walls of the channels is typically similar or even larger than that at the bottom.
Another non-ablative approach that has been considered for such skin treatment is the use of an array of needle electrodes that is first inserted into the skin, and then energized to provide therapeutic effects. Examples of this approach include US 2007/0142885 and US 2008/0091182. However, insertion of the needles and following tissue coagulation in this approach does not involve tissue removal by ablation (vaporization and ejection forming the empty channels or craters), and thus is not as effective in skin tightening as the ablative laser approach.
It would be desirable to be able to ablate tissue and create channels of arbitrary aspect ratio and with an independent control over the width of the thermal damage zone at the side walls and at the bottom.